The present invention relates to a cellular mobile communication system such as vehicle/hand-held telephone systems, cordless telephone systems (PCS), and the like and, more particularly, to a system having a roaming function.
In recent years, along with growing demand for mobile communications and evolution of communication techniques, many cellular mobile communication systems have been built and the areas covered by these systems are expanding rapidly. When a user wants to use a cellular mobile communication system, he or she purchases a terminal device of the desired system, and signs a contract for services provided by the carrier of that system. Under the contract, the user can receive communication services such as mobile voice communications, mobile data communications, and the like using his or her own terminal device everywhere in the service area covered by the system.
Recently, many carriers provide public mobile communication services such as vehicle/hand-held telephone services, a PCS (Personal Communication System), and the like. In addition to these public systems, in establishments such as factories and the like, schools, hotels, office buildings, and the like, private mobile communication systems such as a private PCS or the like are being built to provide their own mobile communication services. Under such circumstances, some users want to use not only the systems to which they belong but also other systems.
In order to meet such demand, in a recently proposed system, the system identification numbers of the individual systems to be used are registered in advance in a terminal device, so that a single terminal device can be used in a plurality of desired systems. With this system, when a user who has a terminal device of a certain public PCS visits the establishment or office building of an allied company or customer, he or she can talk on extension and subscriber lines using the private PCS built in such establishment or office building, resulting in much convenience.
However, in such conventional system, the user must register the system identification numbers of the systems he or she wants to use. For this reason, in an establishment or office the user visits for the first time other than those of allied companies or customers, the user cannot use his or her own terminal device even if he or she need to communicate. In this case, the user must locate a public telephone, resulting in much inconvenience.